I. Field of the Invention
The present invention relates to a magnet for a magnetic resonance imaging device.
II. Description of the Prior Art
Present day magnetic resonance imaging (MRI) devices employ a powerful magnet which may be constructed in one of three basic designs: resistive, superconducting or permanent.
Prior art magnets of the resistive type typically comprise four coils assembled in a psuedo-Helmholtz coil configuration. These four coils create, along their longitudinal axes, a relatively homogeneous magnetic field, the return path for which passes through the air outside of the confines of the coils. Additional shim coils are employed adjacent the four primary coils in order to increase the homogeneity of the resultant magentic field within the coils.
Such prior art magnets have a disadvantage in that the field shape within the coils is determined mainly by the critical location of the Helmholtz and shim coils and by the currents flowing through these coils. Adjustments to coil position and to coil currents are performed manually. In addition, because the returning magnetic field is not contained within such magnets, either a large clear space is required around such magnets or static field shielding is required. Moreover, the power consumption of such magnets is considerable. For a resistive magnet to produce a field strength of 1.5 kilogauss 50 kilowatts of power may be required.
Prior art magnets of a superconducting type are used to achieve higher field strengths for which the power requirements become prohibitive using conventional resistive techniques. However, at higher field strengths the returning field extends over a greater area outside the magnet, making static field shielding even more necessary and cumbersome. With this shielding super-conductivity magnets may weigh as much as 60 tons. In addition, the equipment required to achieve superconductivity greatly increases the complexity and expense of the resultant system.
Permanent magnets have also been employed in MRI devices. Although permanent magnets do not require power and are therefore cheaper, such magnets have the disadvantage of being incredibly heavy. For example, such magnets weigh on the order of 100 tons to achieve a 3.5 kilogauss field. In addition, such magnets can not be turned off. Thus, permanent magnets may require disassembly if a metallic object becomes trapped against an exposed pole piece.
However, permanent magnets for use in MRI devices have the advantage of providing a return magnetic field contained within the magnet and, as a consequence, the magnet is substantially insensitive to external ferromagnetic objects.
To achieve the requisite uniform field between the pole pieces of the permanent magnet, attempts have been made to shape the exposed surfaces of the pole pieces. Specifically, at least one attempt has been made to shape an otherwise flat pole piece of a permanent magnet by employing a plurality of small adjustable screws which extend outward from the pole piece a distance which can be adjusted by setting of the screws.
As a consequence of the foregoing, attempts have been made in the prior art to develop a magnet for MRI devices which employs magnetically conductive metal pole pieces, preferably constructed of iron or an iron alloy, with a magnetic return path also constructed of magnetically conductive material, and which may further employ a pair of activation coils, one located around each pole piece to establish a magnetic field between those pole pieces. This form of magnet has the potential advantage of having a lower manufacturing cost than a Helmholtz air-core type magnet with similar performance.
The materials employed in such a magnet may be inexpensive, for example, simple iron. Since the field shape is not critically dependent on the position of the coil, and since the power dissipation is much lower, the coil manufacturing criteria are much less critical, and, therefore, less expensive, than for an air return path system of the Helmholtz variety. This reduced power dissipation also permits the use of a low cost heat exchanger or possibly no heat exchanger at all.
The weight of such an iron core magnet is anticipated to be on the order of 5-10 tons, assuming a field of 1.5 kilogauss. This compares very favorably with 100 ton for a permanent magnet device and 60 tons for superconductivity type device with shield. Moreover, since the return magnetic field is contained wtihin iron, and magnet is insensitive to external ferro-magnetic objects.
However, use of a non-permanent magnetic material as pole pieces severely limits the available mechanisms for establishing a field of the requisite high degree of homogeneity between the exposed faces of the pole pieces.
It is, accordingly, an object of the present invention to provide a magnet with non-permanent magnet pole pieces surrounded by activating coils and employing a magnetically conductive metal return path which magnet has a high degree of homogeneity in the magnetic field established between the exposed faces of those pole pieces upon activation of the coils.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention.